How to set the default duration of imported images in the VSE
The trick is to add all the images in one go using the Shift+A (Add Image) and Shift+Click each picture or press A if they are all in the same folder.  (Drag and drop doesn't seem to work the same way.)  Once you've inserted the images you will have a strip of one image per frame.  Select the strip and then press 'Y'.  You can then enter a number of frames to use for each image.  The result is a sequence of strips, one per image, all of the same length.  Make sure you have enough space on the current strip to the right of the insertion point to accommodate the new strips before pressing Y.

I'm unaware of any setting for this default value although mine was 26 frames so maybe there is a way.